Monostable multivibrator



United States Patent MONSSTABLE MULTIVIBRATOR Arch C. Luther, .l'r., Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 20, 1%53, Serial No. 343,623

12 Claims. (Cl. 250-27) This invention relates to monostable multivibrators and particularly to such multivibrators exhibiting a high degree of stability in operation.

A monostable multivibrator comprises a pair of intercoupled electron discharge device electrode structures or tubes so arranged as to generate an output square wave having a predetermined duration following the application to the multivibrator of a trigger pulse. The duration of the output square wave is determined by a timing circuit which usually consists of a timing capacitor and a timing resistor in the circuit coupling the plate of the first multivibrator tube to the grid of the second multivibrator tube. The first tube is normally cut 01f and the second tube is normally conducting. The trigger pulse reverses the conditions of conduction as a result of which the second tube remains non-conductive for a period of time determined by the time constant of the timing circuit.

The duration of the output square wave is normally influenced by the characteristics and conditions of the tubes employed and by the value. of the power supply voltages applied to the circuit. It is often desirable to employ a monostable multivibrator which continues to provide an output square wave of a given duration despite changes in the tubes and power supply voltages. It is therefore a general object of this invention to provide an improved monostable multivibrator the output of which is substantially unaffected by changes in the tubes and power supply voltages.

It is another object to provide an improved monostable multivibrator which combines circuit simplicity with stability of operation.

It is a further object to provide an improved monostable multivibrator which combines speed of operation with stability of operation.

In one aspect, the invention comprises a multivibrator circuit having a first electrode structure or tube the plate and cathode resistors of which are connected, respectively, to the positive terminal and the negative terminal, referenced to ground, of a source of unidirectional potential. The plate of the first tube is connected through a timing circuit to the grid of a second electrode structure or tube. The cathode of the second tube is connected to the source of negative potential and the plate thereof is connected to the grid of the first tube and also through a resistor to ground. A diode is connected from the cathode of the first tube to ground to prevent the cathode from falling in potential more than a fraction of a volt below ground when the first tube is cut olf.

Other objects and a better understanding of the invention will appear from a reading of the following description of the invention taken together with the appended drawing wherein:

Fig. 1 is a schematic circuit diagram of a monostable multivibrator constructed according to the teachings of this invention, and

Fig. 2 is a chart showing variations with time of the voltages at various identified points in the circuit of Fig. 1.

2,857,512 Patented Oct. 21, trees ice Referring to Fig. 1, there is shown a monostable multivibrator circuit comprising first and second evacuated electron discharge devices V1 and V2. A negative trigger input terminal 10 is coupled through capacitor 11 to the plate 12 of the first multivibrator vacuum tube or ,electron discharge device V1. Plate 12 is connected througl a plate resistor 14 to a B+ (positive) terminal of 1 source of unidirectional current (not shown). The neg ative terminal of the source 'is connected to ground Cathode 15 of tube V1 is connected through a cathodt resistor 16 to the negative terminal 17 of a second source of unidirectional current (not shown). The positive terminal of the second source is connected to ground. As is understood, the two sources, of unidirectional current mentioned may be a single source having an intermediate or reference point to which the ground connections are completed. Cathode 15 is also connected. to the cathode 18 of a diode V3, which may be athermionic or a crystal diode, having an anode 21 connected to ground. Tube V1 has a grid 22 connected through a resistor 23 to ground.

The plate 12 of first multivibrator tube V1 is connected through a timing capacitor 24 to the grid 25 of a second multivibrator tube or electron discharge device V2. Tubes V1, V2 and V3 may be electrode structures in a common or separate evacuated envelopes. Grid 25 is connected to groundthrough a grid resistor 27. Tube V2 has a cathode 28 connected to the negative'tefrminal 17 and a plate 29 connected to output terminal 30 and via lead 31 to the grid 22 of first tube V1.

In the operation of the circuit of Fig. 1, tube V1 is normally cut 013? and the tube V2 is normally conducting. A current is normally flowing from ground through diode V3 and resistor 16 to negative terminal 17 and this maintains cathode 15 of tube V1 at about one-half of a volt below ground. A negative triggerpulse applied to input terminal 10-passes through capacitors 11 and 24 to grid 25 of tube V2. Tube V2 is thereby cut off, causing a more positive (actually less negative) voltage on plate 29. Since plate 29 1s directly connected to grid '22 of tube V1, tube V1 is rendered conductive and the potential on the plate 12 of tube V1 drops. The drop in potential is coupled through capacitor 24 to grid 25 of tube V2 and tube V2 is maintained cut off following the application of the negative trigger pulse for a period of time determined by the time constant of the RC timing circuit including resistor 27 and capacitor 24. The potential on grid 25 of tube V2 rises exponentially toward ground potential as capacitor 24 charges up due to current flowing from B+ to ground through resistor 14, capacitor 24 and resistor 27. When the potential on grid 25 reaches the cut-off potential, tube V2 starts conducting. The resulting potential drop on the plate 29 of tube V2 is connected tothe grid 22 of tube' V1 causing tube V1 to be cut olf, in turn causing tube V2 to conduct more heavily, all in a rapid regenerative manner. The cycle of operation is then repeated upon application to terminal 10 of the next following negative trigger pulse.

The voltage waveforms at various identified points in the circuit are shown in Fig. 2. It will be noted that the voltage on the plate of tube V1 is at 13+ potential when the tube is cut off and the potential drop to about 158 volts when the tube is conductive. The cathode of tube V1 is maintained close to ground potential when tube V1 is cut off by the flow of current from ground through diode V3 and resistor 16 to negative terminal 17. When tube V1 is conductive, the current flow through resistor 16 maintains the cathode of tube V1 slightly above ground. The plate of tube V2 is returned to ground through resistor 23 and therefore the plate of V2 is at ground potential when tube V2 is cut oflE and is at a lower potential when tube V2 is conductive. The grid of tube V1 is directly connected to the plate of V2 so the voltage waveforms thereon are the saineI Wh'e'iitiib'eVZ The voltage rises" exponentially" toward ground potential capacitor i24 charges up by current flow in the c'lrcult including timingresistor 2 7.

The mul ivi brator crrcmt of Fig. l differs from prior art multivibratorjs cirhdits in the use of adiode V3 and acathode'resistofM, and inl-themari ner ofcross'cbiipling the multivibrato r tithes V1 and'V2 Follow'fiig the input trigger pulse, ft'h'e' n ifiltivibrator piavidesan output pulse having a wi d their duration which' does not vary from the predetermined'value as the tubes age, when a new tube is substituted in" the circuit, drama the power supply voltages vary. 7

As is setfoi'th eirpage 1906f voluinel9; Waveforms 0f the M. I. T Radiation Laboratory Series, the duration ofthe outpuFsQuarewaiiE ofa'r'nonostable rnultivibrator is in accordance with the formula:

T-ln c where E is the initial voltage from which the timing waveform begins, E isthe ultimate voltage which th ttmingfyv aveforrn would res if it" w re permitted to do so, and E is thecritical'voltage'or'ciit offvoltage of tube he t'ra'nsitihn amet terminate the output' square wave. I'rithecircuitbf Figi 1, is fixed a't 'z erovbltsor g'rdtindpiitehtialfby'r'ea's'on' of timing "resistor 21 being returned to ground; The abet/e formula may therefore" be simplified as" follows:

r il e a r' h vane. ee cond ctivity or tuhes with age-and despite U I x en tubes whi-ch may be substituted in the circ it,- then the duration of the output square wave will reniain constant This ideai is approached in the circuit of this invention because E is equal to approximately 2E the voltagedifierence between cathode 15' and grid 22 'of tl lbe V1 when the tube is conducting. Changes in tube Vl cause changes in the voltagedifier- "ence between cathode 15 and'grid 22; but this voltage difference is so small tlfdmpa red with 2E that even large changes in: the voltage diflerence have a negligible effect on the voltage li This advantageous condition results fromthe'construction wherein a cathode resistor 16. is employed between cathode and negative terminal 17 of the source voltage of E Resistor 16 is effective whentube VI is conductjing (during fhe ou tput square wave) iode V3 prevents c athode' ll s oftube from going more than a of a-vol-tbelow .ground' when tube V1 is not conducting wallowing eurrenuo now tromg round through resistor 16 to the negative terminal 17. Therefore onlya m oderate voltage drop devlope d across resistor 23 by the action of tube V2 is needed to cut tube V1: 01f. f V

Thevalue of E- in the circuit of Fig. 1 equal to E plus the voltagedifierence between the cathode 28 and grid 25 of tube V2 when the tubejustPbe'gitis conducting. This voltage difference is small compared with E- and it usually varies only plus or minus one volt in different tubes of the same type. Therefore tube changes have a negligible effect on: the valueo f E Furthermore, small changes in th voltage E will cause arranges in theti'riie t which 'tu be vz shifts ear the non-casduafive condition to the conductive condition. The efieet of these changes are minimized by the" timing waveform steep where it intersects the cut-off yolt ag'e' E' so that" the changes have very little effecton' the time at which "the transition takes place. This is done by making E a n'rgepreefita'ge' of E InFig'. 1, E is'about'fifty'percent of E when plate resistor 14 equals cathode resistor 16. The effects of tube changes on E and E and consequently on the duration of the output square wave can be further minimized by the use of tubes V1 and V2 which have a high amplification factor.

Changes inthe negative bias voltage E from the terminal 17 of the second source have very little efiect on the duration of the output square Wave because the changes in E change both E and-E in such a manner that the ratio E E in the formula above remains practically-constant. Changes in the 13+ voltage have no eflect on tube V2.

rent which-flows through tube V1 thereby maintaining at conducting condition. a

A multivibrator constructed according to Fig. 1 wasfound to provide an output square wave the width; of 1 which-varied from 950 microseconds only plus or, minus the f ollowingpercentages due to the listed causes:

0.75%" due to aging of tube V1 to one-half the normal emission;

0. 0%; Y crystaldiodefrom 20,000 ohms to infinity. v

0E07S% due to plus and minus 5% independent variation s'in the power supply voltages.

0.8% due to plus or m1 ageof tube V2.

Thecircuitlof Fig. l imposesno limitations of any consequence on the sizes of plate resistor 14 and plate resis ton Therefore, thesei resistors may be made as small asj'i's nfecessary to provide the: desired speed of opv efation in} the transitions between the two conditions of theorem 7 I, V I v one? application of the invention, given by way of example only, is in the synchronizing generator- 0f a television transmitter. k V l pulses at repetitionra-tes of 15,750 and 60 cycles per second to control the picture line and field rates. It also provides a standard picture line waveform to which is added the video signal so that the transmitted signal in cludes synchronizing information as well as picture in: formation. The multivibrator of this invention is especially useful in performing a number of the functions in: a television synchronizing generator. I electron discharge In the appended claims the terms device and tube are deemed to include an electrode structure regardless of whether it appears by itself within an individual envelope or with another electrode structure within a common envelope. The term ground is deemed to include-any fixed point of reference potential andis not limited to an actual earth connection.

What I claim is:

l. A monostable multivibrator comprising first and second electron tubes having cathode, grid andplate elecw trade, a source of direct'current potentials including rela-' tively positive, reference and negative terminals, impedance means to return the plate of said second tube to said reference terminal, impedance means to return the cath ode of said first tube to said negative terminal, additional means coupling the electrodes of said tubes and the terminals of said source so that when one of the tubes is conductive, the other is non-conductive, and vice versa, said additional means including a timing circuit coupling the plate of said first tube to the grid of said second tube, and a diode having a cathode connected to the cathode of said first tube and having an anode connected to said.

reference terminal to substantially prevent said cathode of said first tube from. .goingnegative.

Changes in the B+ voltage will normally be a small percentage of the total voltage of the 7 13-!- plus the B applied across the circuit of tube V1. The eftect of cathode resistor 16 is to stabilize the curdue to variation of the back resistance of-V3" nus 30% change in cut-oflEvolt- A synchronizing generator provides multivibrator as defined in claim 1 wherein said? cathode of said diode is connected to the cathode of said first tube over a path capable of passing direct current and said anode of said diode is connected to said reference terminal also over a path capable of passing direct current.

3. A monostable multivibrator comprising first and second vacuum tubes having cathode, grid and plate electrodes, a source of direct current potentials including relatively positive, reference and negative potential termials, a resistor connecting the plate of said firsttube to said positive terminal, a direct current connection devoid of concentrated impedance from the cathode of said sec ond tube to said negative terminal, a resistor connecting the cathode of said first tube to said negative terminal, a diode having a cathode connected to the cathode of said first tube and having an anode connected to said reference terminal, and regenerative circuits coupling the grid of said first tube to the plate of said second tube, and the grid of said second tube to the plate of said first tube, one of said circuits being a timing circuit.

4. A monostable multivibrator comprising first and second vacuum tubes having cathode, grid and plate electrodes, a source of direct current potentials including relatively positive, reference and negative potential terminals, a resistor connecting the plate of said first tube to said positive terminal, a resistor connecting the cathode of said first tube to said negative terminal, a direct current connection from the cathode of said second tube to said negative terminal, a diode having a cathode connected to the cathode of said first tube and having an anode connected to said reference terminal, a connection from the plate of said second tube to the grid of said first tube, and a resistance capacitance timing circuit coupling the plate of said first tube to the grid of said second tube.

5. A monostable multivibrator comprising first and second vacuum tubes having cathode, grid and plate electrodes, a source of direct current potentials including relatively positive, reference and negative potential terminals, a resistor connecting the plate of said first tube to said positive terminal, a direct current connection from the cathode of said second tube to said negative terminal, a resistor connecting the cathode of said first tube to said negative terminal, a diode having a cathode connected to the cathode of said first tube and having an anode connected to said reference terminal, a connection from the plate of said second tube to the grid of said first tube, a timing capacitor connected from the plate of said first tube to the grid of said second tube, and a timing impedance connecting the grid of said second tube to said reference terminal.

6. A monostable multivibrator comprising a first vacuum tube having cathode, grid and plate electrodes, a plate resistor connected to a source of positive potential with reference to ground, a cathode resistor connected to a source of negative potential with reference to ground, a second vacuum tube having a cathode connected to said source of negative potential, a grid and a plate, a resistance capacitance timing circuit connecting the plate of said first tube to the grid of said second tube, a resistor connecting the plate of said second tube to ground, a

connection from the plate of said second tube to the grid of said first tube, and a diode having a cathode connected to the cathode of the first tube and having an anode connected to ground to limit the negative potential on said cathode ofsaid first tube when said first tube is non-conductive.

7. A monostable multivibrator comprising first and second vacuum tubes having cathode, grid and plate electrodes, a timing circuit connecting the plate of said first tube to the grid of said second tube, means coupling the plate of said second tube to the grid of said first tube, a source of voltage referenced to ground having a negative terminal connected to the cathode of said second tube and having a positive terminal, a resistor connected from said positive terminal to the plate of said first tube, a cathode resistorconnected between said negative terminal and the cathode of said first tube, and a diode hav ing a cathode connected to the cathode of said first tube and an anode connected to ground.

8. A multivibrator comprising a pair of electrode'structures each having plate, cathode and control electrodes, a direct current connection between the plate of one structure and the control electrode of the other structure, a capacitor between the plate of said other structure and the control electrode of said one structure, said capacitor forming part of a time-constant circuit, means for applying a unidirectional potential to the plate of said other structure which is positive relative to a point of reference potential, means for applying a unidirectional potential to the cathodes of both structures which is negative relative to said point of reference potential, said positive potential having a value approximately twice the value of the negative potential, and a diode having a cathode connected to the cathode of said one structure and having an anode connected to said point of reference potential.

9. A multivibrator comprising a pair of electrode structures each having plate, cathode and control electrodes, a direct current connection between the plate of one structure and the control electrode of the other structure, a capacitor between the plate of said other structure and the control electrode of said one structure, said capacitor forming part of a time-constant circuit, means for applying a unidirectional potential to the plate of said other structure which is positive relative to a point of reference potential, means for applying a unidirectional potential to the cathodes of both structures which is negative relative to said point of reference potential, and a unidirectional current passing device having a cathode connected to the cathode of said first electrode structure and having an anode connected to said point of reference potential.

10. In a synchronizing generator of a television transmitter, a multivibrator comprising first and second vacuum tubes having cathode, grid and plate electrodes, a source of direct current potentials including relatively positive, reference and negative potential terminals, a resistor connecting the plate of said first tube to said positive terminal, a connection from the cathode of said second tube to said negative terminal, a resistor connecting the cathode of said first tube to said negative terminal, a diode having a cathode connected to the cathode of said first tube and having an anode connected to said reference terminal, and circuits coupling the grid of said first tube to the plate of said second tube, and the grid of said second tube to the plate of said first tube, one of said circuits being a timing circuit.

11. A monostable multivibrator comprising first and second vacuum tubes having cathode, grid and plate electrodes, a timing circuit connecting the plate of said first tube to the grid of said second tube, means coupling the plate of said second tube to the grid of said first tube, a source of voltage having relatively positive, intermediate and negative terminals, means connecting said negative terminal to the cathode of said second vacuum tube, a first resistor connected from the plate of said first tube to said positive terminal, a second resistor connected from said negative terminal to the cathode of said first vacuum tube, said second resistor having a value substantially not less than the value of said first resistor, and a diode having a cathode connected to the cathode of said first tube and having an anode connected to said intermediate terminal.

12. A monostable multivibratorcomprising first and second current conducting devices having first, second and third electrodes, a source of direct current potentials including relatively positive, reference and negative terminals, impedance means to return said third electrode of said second device to said reference terminal, impedance means to return said first electrode of said first device to said negative terminal, additional means coupling 

